Despite the slow progress in experiments, several theoretical proposals have been directed toward the realization of molecular field-free orientation by exclusively using zero-area terahertz pulses [1]}, [2]}, [3]}, [4]}, [5]}. Furthermore, recent theoretical work has shown that a single half-cycle-zero-area terahertz pulse can result in steady molecular orientation long time after the pulse is turned off [6]}. We recently examined a three-state molecular orientation model with an experimentally available single-cycle terahertz pulse [7]}. We found that the theoretical maximum degree of orientation for three populated rotational states can be obtained in the intermediate nonadiabatic limit. The corresponding rotational excitation processes consist of rotational ladder-climbing and simultaneous multiphoton absorption. It is a fundamentally interesting question whether the pure rotational ladder-climbing excitation can independently generate the maximum orientation while suppressing simultaneous multiphoton absorption processes.
